Many current commercial electronic devices usually require different voltage levels as power sources. Thereby, charge pumps are disposed for using existing voltages to generate different voltage levels. The charge pump circuit according to the prior art generally comprises a voltage source, at least a charging capacitor, an output capacitor, a plurality of switches, and a fixed-frequency clock for controlling multiple switches to generate different voltage levels. First, the voltage source and the charging capacitor are connected in parallel using the plurality of switches for charging the charging capacitor to a voltage level. Then, the charging capacitor is connected in series with the output capacitor. After repeated for a plurality of cycles, the voltage across the output capacitor will be raised to a voltage level much higher than the original voltage source. Nonetheless, there are many drawbacks to control the switches of the charge pump using the fixed-frequency clock.
Take the charge pumps applied in displaying devices for example. It is required by advanced portable equipment that the display technology has to meet low power consumption and high performance. In the past several years, mobile display technology has been developed rapidly; the size of displays becomes larger and the resolution becomes higher increasingly. Nonetheless, the power consumption increases as well. In addition, better user interfaces, such as graphical user interfaces and touch control, extend the usage time of mobile displaying devices. This variation demands that mobile displaying devices should be more power saving for corresponding to limited battery capacity. Thereby, the power consumption of the displaying module must be lowered, particularly for mobile displaying devices requiring continuous display of information.
In general, the driving methods of displaying devices can be divided into active and passive driving methods. No matter active or passive driving method, the scan electrode of the driving chip requires a high voltage. In order to simplify the external power circuit, a charge pump circuit is usually disposed in the power circuit. The design considerations include the loading to be driven while driving the scan electrodes. The loading includes the intrinsic capacitor of the liquid crystals and the parasitic effect as viewed by the driving stage. Normally, this loading determines the multiple and switching frequency of the voltage boosting circuit. Under the condition of not influencing the visual effect, a higher multiple and switching frequency will be set. Nonetheless, higher multiple and switching frequency reduce the voltage boosting efficiency and the voltage conversion efficiency, resulting in excess power consumption.
Please refer to FIG. 1, which shows a schematic diagram of the pulses of the scan driving signals in the scan driver according to the prior art. As shown in the figure, the scan driver according to the prior art generates a plurality of scan driving signals (Vg, Vg2, Vg3 . . . Vgn). Because the scan driver according to the prior art is driven by a charge pump circuit of the power circuit having a fixed multiple and a fixed switching frequency for generating the plurality of scan driving signals (Vg1, Vg2, Vg3 . . . Vgn), the output voltages of the charge pump circuit in the voltage conversion time and the voltage holding time of the square wave are identical. Take the scan driving signal Vg1 for example. The output voltages of the charge pump circuit in the voltage conversion time T1 and in the voltage holding time T2 of the scan driving signal Vg1 are identical.
According to the above description, because the charge pump circuit in the power circuit of the displaying device according to the prior art requires a larger multiple and frequency in the voltage conversion time of the square wave of the scan driving signal for raising the voltage to a level rapidly, the voltage boosting circuit is set to a larger multiple and switching frequency. On the other hand, the charge pump circuit of the power circuit only needs to maintain the voltage in the voltage holding time. The high multiple and switching frequency in the voltage holding time result in excess power consumption. Thereby, there is still substantial zoom for improvement for charge pump circuits using a fixed multiple and switching frequency.
Accordingly, the present invention provides a power circuit of displaying device, which reduces the switching frequency or capacitance of the charge pump in the voltage holding time for avoiding excess power consumption.